The present invention relates to a system and method for determining a parameter of a solid object or workpiece, such as a semiconductor wafer, and more particularly relates to a system and method for determining, in real time, the emissivity and/or temperature of the wafer during thermal processing.
Thermal processing furnaces have been widely known and used for many years to perform a variety of semiconductor fabrication processes, including annealing, diffusion, oxidation, and chemical vapor deposition. As a result, these processes are well understood, especially with regard to the impact of process variables on the quality and uniformity of resulting products. Thermal processing furnaces typically employ either a horizontal-type furnace or a vertical-type furnace. For some applications, vertical-type furnaces are preferred because they create less particles during use, thus decreasing the incidence of contamination and wafer waste, they can be easily automated, and they require less floor space because of their relatively small footprint.
Both conventional types of furnaces are designed to heat semiconductor wafers to desired temperatures to promote either diffusion of implanted dopants to a desired depth while maintaining line width smaller than 1 micron, as known, or to perform other conventional processing techniques, such as the application of an oxide layer to the wafer or deposition of a chemical vapor layer to the wafer. The heating requirements of the wafer during processing are known and well understood, and thus are closely monitored.
Conventional vertical-type thermal processing furnaces, such as tube furnaces, are designed to support the processing tube within the furnace in the vertical position. The thermal furnace also typically employs a wafer boat assembly which is mounted to appropriate translation mechanisms for moving the wafer boat into and out of the processing tube. A wafer-handling assembly is deployed adjacent and parallel to the wafer-boat assembly to transfer the semiconductor wafers from wafer cassettes to the wafer-boat assembly. The wafers are then raised into a quartz or silicon heating tube. The tube is then slowly raised to the desired temperature and maintained at that temperature for some pre-determined period of time. Afterwards, the tube is then slowly cooled, and the wafers removed from the tube to complete the processing. A drawback of this processing technique is that it places constraints on the time-at-temperature to which a wafer can be subjected. Conventional vertical furnaces of these and other types are shown and described in U.S. Pat. No. 5,217,501 of Fuse et al. and in U.S. Pat. No. 5,387,265 of Kakizaki et al.
As the critical dimensions for silicon integrated circuits are continuously scaled downward into the sub-micron regime, requirements for within wafer temperature uniformity and wafer-to-wafer temperature repeatability become more stringent. For example, in 0.18 xcexcm technology, the required wafer-to-wafer temperature repeatability is in the order of +/xe2x88x923xc2x0 C.
Pyrometry has been one method of choice for non-contact temperature measurements of a silicon wafer during processing in a thermal processing furnace, but it suffers from known drawbacks. One drawback is that the emissivity of the wafer backside must be known in order to attain accurate temperature measurements. Typically, silicon wafers have backside layers that can drastically alter the spectral emissivity of the wafer through interference effects, which can lead to temperature measurement errors during processing. Furthermore, the emissivity of the wafer is also dependent on the backside surface roughness and wafer temperature. All of these drawbacks make the determination or prediction of wafer emissivity a difficult task
Prior art techniques have attempted to measure the wafer emissivity in situ, that is, within the furnace or heating chamber, in order to measure the temperature of the wafer during processing. One prior art method for determining wafer emissivity is to employ an AC ripple technique, as set forth, for instance, in U.S. Pat. No. 5,310,260. A light source is employed to illuminate the wafer backside within a heating chamber of the thermal processing apparatus. The radiation reflected from the wafer and the source intensity are measured, and the magnitude of the AC components of the source are extracted. The wafer emissivity is then calculated using a ripple equation. A drawback of this approach is that it occurs completely within the heating or process chamber of the thermal processing furnace, and hence it is difficult, if not impossible, to hemispherically and uniformly illuminate the wafer therein. Consequently, it is difficult to accurately determine wafer emissivity, especially in real-time, during processing.
Another drawback of prior art systems is that the heating lamps used to heat the chamber and thus the wafer are also employed to illuminate the wafer. Further, the orientation and position of the heat lamp is fixed in the system. This fixed lamp position makes it difficult to hemispherically and uniformly illuminate the wafer when disposed within the heating chamber. Moreover, the AC ripple generated by the heat lamps is used to determine wafer reflectivity. The combination of the fixed lamp position and the AC ripple often results in inaccurate wafer reflectivity measurements.
Another difficulty associated with determining wafer emissivity and hence temperature during processing is accurately determining the radiation flux within the chamber during processing. This problem arises since stray light, that is, radiation from sources other than the wafer, is reflected onto the pyrometer when measuring the radiation flux within the chamber. This measured radiation value is aggregated with the radiation emitted by the wafer, and employed to determine the wafer temperature. Since the wafer emittance is all that is desired, the pyrometer signal does not accurately measure radiation emitted just from the wafer. Conventional systems cannot accurately and completely compensate for this stray light component, and hence have difficulty achieving the temperature accuracy required by modern manufacturing techniques.
Due to the foregoing and other shortcomings of prior art thermal processing furnaces, an object of the present invention is to provide a system for accurately determining, in real time, the wafer emissivity.
Another object of the invention is to provide a system for measuring and correcting for stray light within the process chamber.
Other general and more specific objects of the invention will in part be obvious and will in part appear from the drawings and description which follow.
The present invention provides systems and methods for determining the emissivity of a wafer during processing in a heating chamber of a thermal processing apparatus. The system and method provide apparatus for determining the reflectivity of the wafer outside of the heating chamber of the thermal processing apparatus, and then determining the intensity of radiation reflected from the wafer when disposed within the heating chamber. The wafer reflectivity determined outside of the thermal processing apparatus (ex situ) is correlated with the intensity of the reflected radiation of the wafer determined within the heating chamber to determine the reflectivity of the wafer within the chamber (in situ). The system then determines the emissivity of the wafer in real time, during processing, from the in situ wafer reflectivity.
The system determines the ex situ wafer reflectivity by measuring the hemispherical directional reflectivity of the wafer prior to thermal processing the wafer within the heating chamber of the thermal processing apparatus. Specifically, the system can generally uniformly and for example hemispherically illuminate a portion of the wafer with radiation from a radiation source, and then measure the intensity of the radiation reflected from that portion of the wafer. Optionally, the system can then measure the intensity of the radiation of the radiation source, and then determine the reflectivity of the wafer from the measured radiation intensity of the wafer and the radiation source.
According to another aspect, the system can determine a ratio of the intensity of radiation reflected from the portion of the wafer and the intensity of the radiation emitted by the radiation source, generate a calibration curve correlating the reflectivity of the wafer with the ratio, and/or optionally determine the reflectivity of the wafer from the calibration curve.
According to another aspect, the system and method maintains the temperature of the wafer inside the heating chamber during the in situ reflectivity measurement at generally the same temperature of the wafer during the ex situ wafer reflectivity measurement.
According to another aspect, the system generally uniformly illuminates a portion of the wafer with an integrating sphere with radiation from a radiation source, collects the radiation reflected from the wafer, measures the intensity of the radiation reflected by the wafer and emitted by the radiation source, determines a selected mathematical relationship between the measured intensity of the reflected radiation and the radiation from the radiation source, and generates a calibration curve correlating the reflectivity of the wafer with the mathematical relationship. The system then determines the reflectivity of the wafer from the calibration curve. The radiation emitted by the radiation source may be modulated by suitable structure, such as a chopper.
According to still another aspect, the system and method determines the intensity of reflected radiation of the wafer within the heating chamber by illuminating the wafer within the heating chamber with a radiation source, and measuring the intensity of the radiation reflected from the wafer within the chamber with a detector. The detector generates an output signal proportional to the intensity of the reflected radiation. The system then correlates the reflectivity of the wafer within the chamber with the intensity of the measured reflected radiation of the wafer within the chamber, and determines the reflectivity of the wafer within the chamber by R=Kxcex94Vw, where R is the reflectivity of the wafer within the heating chamber, K is a constant of proportionality, and xcex94Vw is the intensity of the radiation reflected from the wafer within the chamber.
According to yet another aspect, the system and method determines the constant of proportionality K by sweeping the wafer through the heating chamber of the thermal processing apparatus, and determining the constant of proportionality K from the radiation reflected from the wafer during the sweep and the reflectivity of the wafer determined outside of the heating chamber.
According to an optional aspect, the system and method then calculates the reflectivity of the wafer during processing from the constant of proportionality K, measures the intensity of the radiation reflected from the wafer during processing, and determines the real time wafer reflectivity from the measured reflected radiation and the constant of proportionality. The system then determines the emissivity of the wafer from the wafer reflectivity during processing.
According to another aspect, the system can determine the reflectivity of the wafer as a function of the intensity of the reflected radiation measured within the chamber independent of the position of the wafer within the heating chamber.
According to still another aspect, the system moves or sweeps the wafer through the heating chamber, and optionally without substantially heating the wafer during the wafer sweep, while concomitantly measuring the intensity of the radiation reflected from the wafer within the chamber at one or more wafer positions. The system stores the radiation intensity and associated wafer position during measurement. The system can also measure the radiation intensity of the radiation reflected from the wafer within the chamber at one or more wafer positions, and calculate the constant of proportionality K to facilitate the determination of the reflectivity of the wafer during processing.
According to another aspect, the system and method provides a radiation source to illuminate the wafer when disposed within the heating chamber, and detects or measures the total radiation from the wafer in the heating chamber with a detector as the wafer moves therethrough. The system can optionally correlate the measured total radiation from the wafer with radiation originating from sources other than the wafer, and determine the amount of radiation emitted from the wafer by subtracting the radiation originating from sources other than said wafer from the total measured radiation. The system can then determine the temperature of the wafer during processing from the wafer emissivity and from the radiation emitted from the wafer.
According to another aspect, the system and method determines the reflectivity of a workpiece during processing in a heating chamber of a thermal processing apparatus, by determining directly the reflectivity of the workpiece outside of the heating chamber of the thermal processing apparatus, and determining the reflectivity of the workpiece during processing within the heating chamber of the thermal processing apparatus.
Other general and more specific objects of the invention will in part be obvious and will in part be evident from the drawings and description which follow.